No matter how you identify with the computing aspect of your life being able to take your contacts and calendar on the go can be pretty invaluable, especially if you have more than three friends and your iCal has more colored blocks than a Rubix Cube. But how can you figure out which device(s) on your wishlist will actually work with Mac OS X's built-in syncing app, iSync? Should you dig through the annals of individual manufacturer's support pages? Or perhaps post across multiple forums for someone - anyone - to answer the call of your syncing questions? No I say! Do not go gently into that convoluted mess of neglected support docs and clunky forum systems!
Bad literature jokes and drama aside, Apple maintains a pretty up-to-date list of iSync-friendly devices on their own. If you're definitely looking for a phone that syncs, this list should help make the decision pretty black and white. However, I say this with a word of caution: while this list is thorough and updated fairly often (typically listing new phone models before they hit the street), it isn't 100% complete in that Apple doesn't always list related model numbers. For example: Cingular had a Sony Ericsson w600 available for quite some time, and it was listed at Apple's iSync devices page. I picked up an unlocked w800i a while back (killer non-Smartphone with a great camera, by the way) - a similar phone with a near-identical version of the OS that synced perfectly fine with my Mac, but Apple didn't list the w800 line until that phone officially came to US Cingular stores. Why is anybody's guess, but the point is: if you have your eyes on a phone that is related by model number to a phone on Apple's list, chances are that it should work with iSync just fine. That said, if you aren't too familiar with a phone that isn't on this list, you should probably still hit up a couple forums to make sure before you thrown down hundreds of dollars on a device that might not actually shake hands with iSync (in other words: that's a disclaimer so you don't sue me).
Lastly, for most Smartphone devices which aren't on this list (outside of Symbian, of course), there are always products like PocketMac and the Missing Sync which can handle syncing BlackBerry, Windows Mobile and Palm OS devices.